1. Field of the Invention
The invention relates to injectors of fuel in the form of a mist (known in the art by the name of spray), for oil burners comprising a tube through which coaxial ducts pass, one of which tubes conveys an atomizing fluid and another a liquid fuel, which are then transmitted to this injector. It also relates to burners equipped with such an injector.
2. Description of the Related Art
A wide variety of atomizing or liquid-atomizing devices are known which coaxially comprise a certain number of ducts for conveying one or more liquids to be atomized and one or more atomizing gases under pressure to an outlet orifice for the atomized product.
In effect, means designed to atomize a given liquid under optimum conditions do not necessarily yield the best results for another liquid which has different physical properties, and furthermore, the desired composition of the mist varies from one application to another.
For example, U.S. Pat. Nos. 2,565,696, 3,035,775, 3,533,558, 3,662,960 and 3,805,869 disclose devices intended respectively for spraying concrete, for spraying the components of a polyurethane foam, for atomizing highly viscous starch solutions, for injecting liquid fuels for rocket motors, and for atomizing polyvinyl chloride solutions in a dry atmosphere with a view to converting them into powder; these devices have highly diverse structures so that the flows through the ducts can present specific patterns, for example helical in the case of document U.S. Pat. No. 3,533,559, so as to bring the various components into each other's presence in an optimum way immediately before or at the time that they exit the device.
However, the atomization of liquid fuels in burners also poses specific problems such as those of avoiding flashback and coking of the outlet injector, which means that it is impossible for devices from other fields to be applied directly or simply adapted that it has long been sought to solve using appropriate means.
For example, U.S. Pat. No. 792,265 discloses a burner which coaxially comprises a central duct through which a liquid fuel flows, and two annular ducts through which a stream of pressurized vapour flows, these ducts being fed in their upstream region respectively with the liquid to be sprayed and with the vapour; the free (downstream) end of the first annular partition which is common to the central duct and to the immediately adjacent annular duct is very much upstream of the free (downstream) end of the second annular partition; thus, initial external atomization of the liquid fuel is achieved when the jet of liquid is sheared from the outside by the vapour arising in an annulus around this jet at the nozzle-forming free end of the first annular partition; the second annular partition delimits internally, starting from this free end, another central duct through which the stream resulting from the first atomization passes; this stream is subjected to a second external atomization at the free ends, located in the same transverse plane, of the two nozzle-forming partitions of the second annular duct. With this structure, it is highly difficult to obtain a mist with precise characteristics.
U.S. Pat. No. 1,279,315 discloses a burner of a similar structure but in which the first annular duct has a stream of air passing through it while the second annular duct has a stream of air or vapour passing through it, with the same drawbacks as those mentioned earlier.
It was thought that by conveying a liquid fluid to be atomized in an annulus around a first, gaseous, atomizing fluid, the atomization process could be better controlled.
Thus, there are also known liquid-atomization devices which comprise, coaxially, from the inside towards the outside, a central duct through which an atomization gas passes, an annular duct through which the product to be atomized passes in the liquid state, and an annular duct through which an atomization gas also passes, these ducts being fed, in a first end region (upstream region) with the atomization fluid and fluid to be sprayed, and comprising respective nozzles in the second end region (downstream region) for spraying the product which was initially in the liquid state, in the form of droplets in suspension in the atomizing fluids.
This is the case in particular for the subjects of documents GB 672,441, EP 0,105,493 and EP 0,593,171, but here too, the prior art is not entirely satisfactory for atomizing a liquid fuel in the charging opening of a furnace.
The problem is that in these three documents the respective downstream ends at least of the two annular ducts end in a nozzle, in which the downstream end of the central duct, which is set back slightly inside the intermediate annular duct, is nonetheless very close; what this means is that although the product in the liquid state that is to be atomized is transmitted through the intermediate annular duct, the atomization caused by the internal atomization gas can be likened to external atomization (it results from the shearing of the jet of liquid by the angular end of the exterior wall of the annular duct for the liquid); the atomization caused by the external atomization gas is naturally also external atomization, and thus these devices may be considered as being devices with two stages of external atomization. In consequence, fine control of the state of atomization of the mist (mean diameter of the droplets and proportion of small droplets) is therefore very tricky.
What this means is that despite the existence of the devices in these three documents, there was still the need to create a device which, on the one hand, could bring about internal atomization and, on the other hand, could bring about external atomization of the liquid to be sprayed.
To this end, in a known type of oil burner comprising an atomization device according to patent document FR 2,737,138, there is provided a tube comprising an inlet part comprising coaxially, from a central duct for a fluid for the "internal" atomization of the oil, and an annular duct for the oil, and an annular duct for a fluid for "external" atomization; between the exterior wall of the tube and the partition of the burner is defined an additional annular duct conveying an oxidizing gas. In the outlet part of the tube but markedly upstream of its outlet end, the central duct for the internal atomization fluid terminates in an "internal" injector opening into the duct for the oil and thus forming, downstream of the internal injector, a central confinement duct constituting a preatomization space for the internal atomization fluid and the oil, surrounded by the annular duct for the external atomization fluid, while the duct for the oxidizing gas remains between the exterior wall of the tube and the partition of the burner.
All or some of the preatomization space may be produced in a component in the form of a "mixture" injector through which the oil atomized by the internal atomization fluid is injected into the charging opening of the burner, and around which the external atomization fluid is injected.
The invention relates more specifically to such a fuel mixture injector through which an internal atomization fluid and the actual fuel (liquid) pass, and therefore aims to create an injector of fuel mist which makes it possible to improve the quality of atomization on the one hand, while at the same time avoiding the presence of "grains" within the flame produced by the burner, the term "grains" denoting fuel droplets which are large enough to remain individually clearly visible as they burn within the flame, and on the other hand while at the same time increasing the flame stability, attachment of the flame at the tip of the burner being made almost systematic.
It might be hoped that this objective could be achieved by reducing the mean time needed to evaporate the population of droplets, by increasing the proportion by mass of small droplets (diameter smaller than 20 .mu.m) within the mist and by decreasing the mean diameter of all the droplets (for constant atomization fluid and liquid fuel flow rates).
In point of fact, in the mixture injector of the device known from document FR 2,737,138, which may be laid out as depicted in FIG. 1, the longitudinal central confinement duct constituting the preatomization space has two regions 11, 12 of different diameters connected frustoconically in a zone 13, the region 11 of larger diameter constituting the inlet region of the injector and the region 12 of smaller diameter its outlet region.
In general, the ratio of the length of the region 12 to its diameter is of the order of 8 to 12 and typically equal to approximately 9.
In this injector of fuel mixture in the form of a mist, it may be observed that a film 21 of liquid fuel of approximately constant thickness is formed along the entire length of the large-diameter region 11 of the central duct, this film 21 being connected to a film 22 extending along the small-diameter region by a frustoconical zone 23; however, the internally frustoconical shape of the film extends beyond the frustoconical connection between the two cylindrical regions of the duct, and leads to the formation of an increased thickness of the film in the small-diameter region, the thickness of the film 22 in this region then decreasing as far as the free end of the injector.
It would seem that under these conditions, the atomization of the liquid fuel results from the detachment of the film which lines the wall of the duct of the injector, which is something which, on the one hand, produces relatively large droplets and, on the other hand, rapidly entrains the small droplets into the stream of atomization fluid (for example air) travelling at high speed, and this limits the amount of fuel evaporated locally and does not allow good combustion of the heavy fuel oil.
The object of the invention is therefore to overcome these drawbacks by increasing the proportion by mass of small droplets and by decreasing the mean diameter of the droplets and the speed of the small droplets, and also to improve the stability of the flame inside and outside the burner charging opening through the adoption of an appropriate geometry for the injector, or at least for the downstream part thereof which terminates at its free end.